This invention relates to the technical field of an image processing method for use in digital photoprinters and the like which obtain digital image data from an optically recorded image by, for example, photoelectrically reading the image recorded on a film, or which directly obtain digital image data by recording the image of a subject and which obtain an image reproducing print (for photo) on the basis of the digital image data. The process is particularly intended to correct various defects such as lateral chromatic aberration, distortion, decrease in the brightness of the edge of image field and a defocused image that develop in the image recorded with a film with lens, an inexpensive compact camera or a digital camera and so forth. The invention also relates to the technical field of an image processing apparatus that makes use of this image processing method.
Currently, the image recorded on a photographic film such as a negative film or a reversal film (which are hereinafter collectively referred to as "film") is printed to a light-sensitive material (photographic paper) by "direct exposure" (or analog exposure), in which the image on a film is projected onto the light-sensitive material to perform areal exposure.
A printer that relies upon digital exposure has recently been commercialized. Called a "digital photoprinter", the apparatus operates as follows The image recorded on a film is read photoelectrically; the image is then converted to a digital signal and subjected to various image processing steps to produce recording image data; a light-sensitive material is exposed by scanning with recording light modulated in accordance with the image data, whereby an image (latent image) is recorded; and the necessary processing is done to produce a (finished) print.
In the digital photoprinter, the image recorded on a film is converted to digital image data, which is then processed to determine the conditions for exposure to be performed in printing; hence, the correction of drop-outs or blocked-ups in image due to photography with rear light or an electronic flash, sharpening, the correction of color or density failure, the correction of under-exposure or over-exposure or the correction of insufficiency of the brightness of the edge of image field can be effectively performed to produce prints of very high quality that have heretofore been unattainable by conventional direct exposure. What is more, by image data processing, a plurality of images can be assembled into one composite image or a single image can be divided into more than one image and even characters can be assembled; thus, prints can be outputted after editing/processing is done in accordance with a specific use.
A further advantage of the digital photoprinter is that prints can also be prepared from the image (image data) taken with an imaging device such as a digital camera. The desired image can not only be outputted as a print (photo); the image data can also be supplied to a computer or stored in a recording medium such as a floppy disk; thus, the image data can be utilized in various applications other than photography.
Having these capabilities, the digital photoprinter is basically composed of a scanner (image reading apparatus) for photoelectric reading of the image recorded on a film, an image processing apparatus that performs image processing on the thus read image to produce recording image data (conditions for exposure) and a printer (image recording apparatus) that performs scan exposure on a light-sensitive material in accordance with the image data and which performs development and other necessary steps to produce a print.
In the scanner, reading light issuing from a light source is allowed to be incident on the film to produce projected light carrying the image recorded on the film and the projected light is focused on an image sensor such as a CCD sensor, whereby photoelectric conversion is effected to read the image from the film; after optionally being subjected to various image processing steps, image data from the film (image data signal) is sent to the image processing apparatus.
The image processing apparatus sets the conditions for image processing on the basis of the image data that has been read with the scanner; the apparatus then performs image processing on the image data in accordance with the thus set conditions and supplies the printer with output image data (exposure conditions) for image recording.
In the printer, if it is of a type that relies upon exposure by scanning with light beams, the latter are modulated in accordance with the image data sent from the image processing apparatus and the light-sensitive material is exposed by two-dimensional scanning to form a latent image, which in turn is subjected to development and other specified processing steps, thereby producing a print (photograph) reproducing the image recorded on the film.
When the image recorded on a film or taken with a digital camera or a compact camera is reproduced on a print, various factors come into play to deteriorate the image quality and they are aberrations that are caused by the poor performance of the lens fitted on the camera used to record or take the image, such as lateral chromatic aberration, distortion, decrease in the brightness of the edge of image field and defocusing of the image.
A color image is formed of three primary colors, red (R), green (G) and blue (B) and due to the slight wavelength-dependent differences in the refractive index of the lens, the imaging magnification of the three light components, R, G and B, varies to cause "lateral chromatic aberration". As a result, the image recorded on a film will have a color mismatch upon reproduction.
In order to record an appropriate or satisfactory image, a plane of a scene of interest normal to the optical axis must be imaged on an identical plane or imaging plane that is normal to the optical axis. In fact, however, ordinary lenses have an aberration called "distortion", or displacement of the imaging plane with respect to the optical axis, and the resulting distortion of the image on the imaging plane causes a problem in that the image recorded on a film is distorted upon reproduction.
Other causes of image deterioration that depend on the performance of the lens used are the decrease in the brightness of the edge of image field, or the image from either edge of the lens being darker than that from the central area, and defocusing, or the production of a blurred image due to the varying focus position across the plane of the film.
If comparatively high cost can be spent to manufacture cameras as in the case of a single reflex lens camera, high-precision lenses may be employed and optionally a plurality of lens elements may be combined to record an appropriate image on the film by correcting various lens aberrations including lateral chromatic aberration, distortion, decrease in the brightness of the edge of image field and defocusing of image.
On the other hand, a film with lens and an inexpensive compact camera do not justify high cost and lens aberrations such as lateral chromatic aberration and distortion will occur in the image recorded on the film. As the result, the image reproduced in a print will have various defects such as color mismatch, distortion, unevenness in brightness and blurring.
FIG. 13 shows how the image on a film is processed with a conventional digital photoprinter and outputted as a finished print. As shown, the image on a film is scanned with a scanner to read high-resolution, fine scanned image data, which is subsequently subjected to predetermined image processing and setup steps in an image processing apparatus; this is not followed by image displaying but the processed image data is immediately fed into a printer which outputs a finished print. If the image on a film that involves aberrations such as lateral chromatic aberration and distortion is processed by the stated procedure, the aberrations are not effectively corrected and the reproduced image on the print will have a color mismatch and distortion.
As also shown in FIG. 13, print outputting from the conventional photoprinter is preceded by prescanning of the image on a film with the scanner to produce low-resolution, prescanned image data, which is displayed as such on a monitor (i.e., an image display device); alternatively, fine scanning is directly performed in place of prescanning so as to produce high-resolution, fine-scanned image data, which is displayed on the monitor after removing portions of it or effecting size reduction. Whichever is the case, aberrations such as lateral chromatic aberration and distortion in the image data are not corrected and the image displayed on the monitor also has a color mismatch and distortion.
To deal with this problem, the user may verify the color mismatch and distortion in the displayed image and manually correct the lateral chromatic distortion and distortion. However, positively verifying these aberrations from the displayed image and manually applying the appropriate correction to the image data are extremely difficult even for a skilled user; even if aberrations are successfully corrected, a phenomenon called vignetting to be described later will occur in the corrected image, and it is extremely difficult to achieve correct verification of the range of vignetting that results from the correction of aberrations. What is more, in order to produce a print that is properly corrected for the aberrations and which has an appropriate print area reproduced thereon, print outputting must be repeated with small variations in the amount of correction; however, this is quite cumbersome and, in addition, the printing cost is increased.
If the information about the lenses used to take a picture of a subject or their characteristics are known in the conventional image processing apparatus, it would be possible to correct lateral chromatic aberration, distortion and other kinds of aberrations in accordance with the characteristics of the taking lenses. In fact, however, there is no established method of correcting aberrations and if one wants to perform the actual correction, many trials and errors are necessary before the appropriate result is attained and this has been a quite cumbersome job. If the information about the taking lenses or their characteristics are not known, it is even more difficult to correct the aberrations and producing the most effectively corrected image has been extremely difficult.